The use of human or animal tissue for medical or surgical use is a rapidly growing therapeutic field. Many uses of processed biological tissues for implantation into humans have been reported. The commercial products or products under development include wound healing dressings, tissue heart valves, ligament substitutes, pericardial patches and membranes, vascular grafts and the like. The use of animal tissue offers an inexpensive source of materials to fabricate tissue-based medical products. The problems with the animal tissue transplantation include inflammation, unwanted degradation, control over the degradation process, calcification, inability to release bioactive compounds in a controlled manner, and rejection of the transplanted tissue.
The primary component of many biological tissues is a protein called collagen. Collagen-based biomaterials generally induce a mild inflammatory response, which results in degradation of the protein. This degradation can be prevented by chemical modification or crosslinking of tissue proteins and is achieved by reacting difunctional and polyfunctional crosslinkers capable of forming irreversible and stable intermolecular chemical crosslinking between two collagen chains. Chemical crosslinking may also increase strength and durability of the tissue. Many heart valve bioprosthesis manufacturers use glutaraldehyde as a crosslinking agent for stabilization of the bioprosthesis tissue. The chemistry of glutaraldehyde is complex but well documented. Glutaraldehyde reacts with free amine groups from lysine residues on collagen and forms Schiff base addition products. Although glutaraldehyde is the most commonly used chemical fixative for biological tissues, there are a number of drawbacks associated with its use in the production of bioprosthetic devices. For example, the long term durability of glutaraldehyde-fixed bioprostheses is not well established. Another drawback to glutaraldehyde fixation of bioprostheses relates to the release of cytotoxic glutaraldehyde on the tissue surface thereby hindering the growth of cells, especially endothelial cells, on the surface of the tissue. Glutaraldehyde fixed tissue is also susceptible to calcification which leads to device failure.
To overcome limitations of glutaraldehyde crosslinking, other chemical crosslinking agents capable of reacting with amine, carboxyl and hydroxyl group have been explored. Tissue crosslinking chemistry has been recently reviewed. However, none of alternative chemistries have resulted into a commercial clinical heart valve product. Tissue crosslinking chemistry is challenging due to a variety of reasons. From the chemistry point of view, the crosslinking reaction is a heterogeneous reaction, where the reactant (tissue) is always in a separate phase (solid state phase) as compared to the crosslinker (solution, oiled or liquid phase). This limits the accessibility of tissue functional groups for crosslinking reaction. The solid state nature of tissue also makes it difficult for large crosslinker molecules such as, by way of example, and not limitation, polymers to penetrate inside the tissue matrix and crosslink the reactive sites. Generally, the crosslinking reaction must be done without denaturing the protein. The denatured tissue/collagen (gelatin) is more susceptible to enzymatic degradation and denatured proteins have inferior mechanical properties as compared to non-denatured tissue. To prevent denaturing of tissue, the use of aggressive organic solvents and high temperatures in tissue crosslinking is generally avoided. It is generally believed that an aqueous medium with physiological conditions (pH 7.2, 37° C.) is best suited for tissue crosslinking. Fixation under physiological conditions is most likely to preserve the natural conformation of proteins present in the tissue. Glutaraldehyde is one of the few crosslinking agents capable of reacting with the tissue in water under physiological conditions.
In known approaches, most of the tissue crosslinking is restricted to di- or polyfunctional small compounds such as, by way of example, and not limitation, glutaraldehyde. Small compounds can easily penetrate solid tissue matrix and can crosslink surface as well as bulk components of the tissue matrix. In order for crosslinking to occur, two or more reactive functional groups must react with two polymeric chains to form an interchain crosslinked moiety. Most tissue crosslinkers are single chemical entities and therefore have fixed molecular length. The fixed length crosslinker can only react with those sites which are within the close proximity of its reactive functional groups. Therefore, it cannot crosslink the tissue if the reactive sites present on the tissue are at a shorter or longer distance than the length of crosslinker. Also, during the crosslinking reaction, one of the crosslinking functional group reacts with the crosslinkable moiety such as, by way of example, and not limitation, collagen. After the reaction, the other functional group must react with other reactive site on the collagen to complete the crosslinking reaction. This often may not be possible due to limited length and mobility of crosslinker. This results in a number of dangling bonds with incomplete crosslinking. Therefore, the length of a crosslinker serves as a major limitation in achieving a high degree of crosslinking. Thus, there is a need for tissue crosslinking methods wherein the crosslinks formed may have variable lengths.
Polymeric crosslinkers can be useful in crosslinking the tissue due to high molecular flexibility of polymeric molecular coil and polymer's ability to impart additional properties to the tissue matrix. However, polymeric crosslinkers are large molecules which cannot diffuse/penetrate inside the tissue matrix and react with sites present in the bulk of the tissue. This limits the ability of polymeric crosslinker to surface crosslinking only. Known techniques generally do not teach the successful use of polymeric crosslinkers in tissue crosslinking. There is a need for methods and compositions that permit the incorporation and crosslinking of tissue using polymers or that generate polymeric crosslinks.
Shape memory biomaterials have the ability to change to a predetermined shape when subjected to an appropriate energy stimulus. Nitinol alloy is one of the well-known shape memory biomaterials. Many applications of Nitinol materials have been commercialized. These applications include peripheral vascular stent and stent grafts, vena cava filters, etc. Bioprosthetic tissues having shape memory properties can be extremely useful in making novel medical devices. There is a need for tissue-based biomaterials that can remember shape maintained during fixation or stabilization and tissue-based materials with the ability to remember and recover the shape when deformed by a mechanical force.
Unfixed or non-crosslinked animal tissue undergoes enzymatic degradation when implanted in human/animal body. Usually such degradation is followed after a moderate to severe inflammatory response; presumably due to an immunological reaction to the foreign biological materials in the host body. Non-crosslinked animal tissue such as, by way of example, and not limitation, porcine small-intestinal submucosa has been commercialized as a wound dressing material. In many medical applications, it is desirable to have a biological degradable tissue with no or little inflammatory response and control over its degradation profile and properties. Known techniques generally do not teach methods and compositions that will affect the degradation behavior of biological tissue. Therefore, there is also a need for methods and compositions that can reduce the inflammatory response to the animal or human tissue. Compositions and methods that will control the degradation time of the implanted tissue are also needed.
Animal tissue used in commercial bioprostheses such as heart valve, vascular graft and vascular patch is limited by tissue thickness, size and protein (chemical) composition. For example, bovine pericardium, a widely used animal tissue has a thickness ranging 1 to 2 mm which may too thick for some medical applications such as low profile stent graft application. The useful tissue recovered from one animal is also limited in size. Typical area of bovine pericardial or porcine pericardial tissue may range from 50 to 150 square inches. This size and thickness limitation may limit the use of tissue in making large medical device such as tissue based dialysis catheters. The tissue size limits also increases production costs due to lower yields. The higher size of implantable tissue may permit to manufacture more devices per tissue and reduce manufacturing costs. Therefore there is need for tissue, especially membrane like tissue, which can be made in wide ranges of size, thickness and with different chemical compositions for bioprosthesis applications.
Synthetic biodegradable polymers have received considerable interest in the medical and pharmaceutical field at least because they can perform temporary therapeutic functions and are eliminated from the body once their therapeutic function has been accomplished. Some of the well-known applications of biodegradable polymers include surgical sutures, staples, or other wound closure devices, as a carrier for bioactive substances for controlled drug delivery, etc. Several types of biodegradable polymers have been reported in the subject literature, however, polymers prepared from hydroxy acids have received much attention due to their degradability and toxicological safety. Homopolymers and copolymers based on the l-lactic acid, dl-lactic acid and glycolic acid are among the most widely used polymers for medical applications. These polymers can be formulated into variety of physical forms such as, by way of example, and not limitation, fibers or filaments with acceptable mechanical properties and degradation profile and nontoxic degradation products. Synthetic biodegradable polymers such as, by way of example, and not limitation, polyanhydrides, polylactones, and polyhydroxyacids have been extensively investigated for controlled drug delivery applications as well as for a scaffold for tissue engineering. These polymers can release a bioactive compound upon bioerosion and thus permit localized controlled therapeutic delivery. There is a need for biological tissue, preferably degradable biological tissue, which can release a bioactive compound in a controlled manner, preferably using a hydrolysis or bioerosion mechanism. There is also a need for materials which can provide properties of synthetic biodegradable material and biological tissues.
Polyethylene oxide (PEO) or polyethylene glycol (PEG) is a water soluble biocompatible polymer which is being used in several commercially available pharmaceutical and medical products. PEG is water soluble and non-ionic in nature. When injected in human or animal body, it is rapidly cleared by the body. When it is immobilized either physically or chemically on a polymer surface, it renders the surface highly resistant to protein adsorption. The resistance to protein adsorption is believed to be responsible for reduced bacterial and cell adhesion to PEG-rich surfaces. The reduction in protein adsorption also increases the biocompatibility of blood- and tissue-contacting materials. Hydrated PEG chain is not recognized by the immune system, therefore it is used to reduce the immunogenicity and antigenicity of proteins and hence increase their circulation time. Nonionic hydrogels, such as, by way of example, and not limitation, the poly(ethylene glycol) (PEG)-based hydrogels, are biocompatible and are non-cell adhesive. Tissue-based bioprostheses which combine the properties of PEG and biological tissue may be useful for many medical applications.
In view of the foregoing, there is a need for compositions and methods that provide biostable implantable tissue. There is also a need for biodegradable biological tissue with control over its degradation time and with the ability to release bioactive compounds.
Unless otherwise indicated illustrations in the figures are not necessarily drawn to scale.